Leases - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		12 Months Ended
	Jul. 31, 2015	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2016	Dec. 31, 2018
Operating Leased Assets [Line Items]
Operating lease costs (gross lease expense)		$ 3,000	$ 900
Variable Lease Costs		1,500
Sublease Income		$ 1,800
Operating lease, Weighted average remaining operating lease term		9 years 6 months 10 days
Operating lease, weighted average discount rate, percent		7.70%
Financing obligation on leased facility		$ 0			$ 92,009
San Mateo Facility
Operating Leased Assets [Line Items]
Operating lease expiration period	Dec. 31, 2028
Capitalized construction costs				$ 71,800
Leasehold improvement costs reimbursed by landlord				$ 14,300
Financing obligation on leased facility					$ 92,000